Orange pigment change

Excess N can block Calcium uptake

A calcium deficiency would manifest itself by weakened stems, brown spots on the upper fan leaves and in areas exposed to light... but doesn't curl the leaves. Are you over or under watering? This could account for the curling leaves - but not the color change. Your grow looks so good, I guess I wouldn't worry too much about the coloring unless it progresses. If it continues, I'd start to think a phosphorous deficiency which works from the leaf tips in and turns the leaves a nasty color (yellow, brown, purple)... it, too, will result in leaves curling. Good luck with your grow!

Plants need more calcium.